There are numerous commercial jet aircraft of various sizes including the large "jumbo jet" aircraft, such as the Boeing 747 and the McDonnell Douglas MD11 and the Lockheed L1011. In going to a still larger aircraft, such as a 600-passenger aircraft envisioned for the future, the loads on the wing member needed to hold the airplane aloft are heightened some. These large aircraft will carry in the neighborhood of 600 passengers and may include two passenger decks. Whereas a Boeing 747 (one of the largest commercial jet aircraft in use) has an empty weight in the neighborhood of about 399,000 pounds, it is estimated that the high capacity aircraft envisioned will weigh in the neighborhood of 532,000 pounds empty and somewhere around 1,200,000 pounds loaded. High capacity aircraft as used herein refers to an aircraft weighing more than 450,000 pounds empty. To heighten efficiency in such an airplane, it would be important to have materials in the wing structures that can support the load of the airplane without themselves becoming too heavy. Aluminum alloys have seen wide use in airplane structural members, including airplane wing structural members, and have an enviable record for dependability and performance. More exotic, composite or other materials can be used for airplane wing structural members, but are much more costly and can be somewhat less dependable than aluminum alloys.
In general, the structural core of a large airplane wing can include a box-like structure made up of an upper wing skin, a lower wing skin, and end pieces to close the box-like beam structure. While the upper and lower members are labeled "skin", it is important to appreciate that these are not thin skins such as on the airplane fuselage, but rather, somewhat thick, for instance a half inch or more in thickness. In most of the current commercial jet aircraft, the upper wing skin is made of a 7000 Series alloy, currently a 7X50 alloy (7X50 is intended to refer to 7050 and 7150), or more recent alloy 7055. U.S. Pat. No. 3,881,966 describes 7X50 alloys and U.S. Reissue Pat. No. 34,008 describes 7150 alloy used as an upper wing skin on a commercial jet aircraft, and U.S. Pat. No. 5,221,377 describes alloy 7055 and refers to its use in airplane structural members. The upper wing skins were normally in artificially aged tempers such as T6-type or possibly T7-type tempers. U.S. Pat. Nos. 4,863,528, 4,832,758 and 4,477,292, along with U.S. Pat. No. 5,108,520, all describe tempers for 7000 type aluminum alloys, which said temper can be applied to the 7000 Series alloys just mentioned to improve performance. All the aforesaid patents (U.S. Pat. Nos. 3,881,966, Re. 34,008, 5,221,377, 4,863,528, 4,832,758, 4,477,292 and 5,108,520) are fully incorporated herein by reference.
In commercial jet aircraft, the lower wing skins have generally been made of aluminum alloy 2024 or similar products such as alloy 2324 which is included in U.S. Pat. No. 4,294,625, the entire content of which is incorporated herein by reference. The temper was normally T3-type such as T351 or T39. Temper and alloy designations used herein are generally those used in accordance with the Aluminum Association and are generally recognized in the art and described in the Aluminum Association Standards and Data book.
Both the upper and lower wing skins are often reinforced by stringer members which can have a channel or J-type shape or other shape which are riveted to the inside surfaces to stiffen the wing skins and thereby stiffen the wing box structure. In general, when a commercial jet aircraft is in flight, the upper wing skin is in compression, whereas the lower wing skin is in tension. An exception occurs when the airplane is on the ground where these stresses are reversed but at a much lower level since at that point the wing outboard of the landing gear pretty much just holds up its own weight. Thus, the more important applications are when the airplane is in flight which places the upper wing skin in compression and the lower wing skin in tension. An exception occurs in certain military airplanes which are designed to utilize their enormous power to weight ratio and are intended to fly upside down, right side up, or any condition between at enormous speeds.
Because of the particular loading differences encountered in commercial jet airplanes, the alloy selections were, for the most part, as just described. There have been some exceptions in that airplanes such as the Lockheed L1011 included 7075-T76 lower wing skins and stringers and the military KC135 fueler airplane included 7178-T6 lower wing skins and stringers. Another military airplane, the C5A, used 7075-T6 lower wing skins that were integrally stiffened by machining out metal. Military fighter planes such as the F4, F5E, F8, F16 and F18 have included lower wing materials of 7075 alloy or related 7475 alloy (F16 and F18). Nonetheless, over the years the airplane wing box structure has, for the most part, in commercial jet aircraft included a 7000 Series alloy upper wing skin and a lower wing skin of 2000 Series alloy, namely, 2024 or a member of the 2X24 alloy family.
The important desired properties for a lower wing skin in a high capacity and new commercial passenger jet aircraft include a higher strength than 2X24 alloys, a better fatigue life and improved fracture toughness over 2X24 materials. Because the airplane flies at high altitude where it is cold, fracture toughness at minus 65.degree. F. has become a concern in new designs. Additional desirable features include age formability whereby the material can be shaped during artificial aging, together with good corrosion performance in the areas of stress corrosion cracking resistance and exfoliation corrosion resistance. Alloys used to date for lower wing skin members in commercial jet aircraft are all lacking in satisfying the needed levels for high capacity aircraft in one or more of these properties.
In the past, the wing box-like structure was often made from several pieces, as shown in FIG. 3, left side, wherein the end member or spar comprised a plate fastened to the upper and lower wing skins by riveting to angle or tee-like members, in turn riveted to the wing skins. Some builders prefer to make the entire end piece or spar as shown on the right side of FIG. 3 thus eliminating several rivets and considerable weight by reducing the amount of metal structure. This end piece or spar is made by machining from a thick plate but the plate needs good properties in the transverse directions as well as longitudinal direction.